Practical Presentation: Study of Linkage Maps Based on Data from Drosophila Crosses
Analyzing Gene Linkage
and Mapping in Drosophila melanogaster
Introduction
to Linkage Maps
Linkage maps are representations of the relative
positions of genes on a chromosome, based on recombination frequencies.
Purpose: Used
to study how genes are inherited together.
Genetic Linkage: Genes located close together on the
same chromosome are likely to be inherited together.
Recombination: Crossing over during meiosis can
separate linked genes.
Recombination Frequency: Used to estimate the
distance between genes (measured in map units or centimorgans).
Drosophila
Crosses Setup
Parental Traits:
Parent 1:
Normal wings and red eyes (dominant).
Parent 2: Vestigial wings and white eyes
(recessive).
F1 Generation: Offspring show dominant traits
(normal wings, red eyes).
Test Cross: F1 offspring crossed with homozygous
recessive flies (vestigial wings, white eyes).
Generation Phenotypes
Expected Phenotypes in the F2 Generation:
Parental types (e.g., normal wings, red eyes).
Recombinant
types (e.g., normal wings, white eyes; vestigial wings, red eyes).
Observation: The number of recombinants indicates
how often crossing over occurs between the linked genes.
Calculating Recombination Frequency
Recombination Frequency Formula:
Example:
Total Offspring: 1000
Recombinants:
150
Recombination
Frequency = (150 / 1000) × 100 = 15%
Interpretation:
The genes are 15 centimorgans apart.
Constructing the Linkage Map
Process:
1) Collect
recombination frequencies between multiple genes.
2) Arrange
genes on the chromosome based on these distances.
Example of Map:
· Gene Pair 1: Wing Shape (A) and Eye Color (B)
- Recombination Frequency: 15% → Distance = 15 cM
· Gene Pair 2: Eye Color (B) and Body Color (C)
- Recombination Frequency: 10% → Distance = 10 cM
· Gene Pair 3: Wing Shape (A) and Body Color (C)
- Recombination Frequency: 25% → Distance = 25 cM
With these distances, you can infer the order of the genes:
- Wing Shape (A) → Eye Color (B): 15 cM
- Eye Color (B) → Body Color (C): 10 cM
- The total distance between Wing Shape (A) and Body Color (C) is 25 cM.
Thus, the order of genes on the chromosome is likely to be:
Map Construction: Based on relative distances.
Interference and Coincidence
Interference: A crossover event in one region of a
chromosome can reduce the probability of another crossover occurring nearby.
Effect on Mapping: Affects recombination frequencies,
so adjustments may be needed to refine maps.
Practical Applications of Linkage Maps
1. Understanding gene inheritance patterns.
2. Used in
agriculture and animal breeding for trait selection.
3. Helps in
studying genetic diseases and traits.
